This invention relates to Optical Encoders that are of common use for electrical motors or other rotating devices. Examples of such optical encoders can be found in U.S. Pat. No. 4,268,747 by Becchi et al. (1981), and U.S. Pat. No. 4,410,798 by Breslow (1983).
Ever since their invention, Optical Encoders have been widely used as position feedback devices for rotary shaft position control in robots, automatic machines and other assemblies.
Known encoders use a rotating optical disc fixed on the motor shaft, the disc having a pattern of sections of alternating optical properties (for example slots having transparent and opaque sections). The disc is placed on the path of an optical beam, between a light emitter and a light sensor. The light sensor then creates an electronic signal the amplitude of which will change periodically according to shaft position. An electronic circuit is then used to count the number of periods of the electronic signal, thus providing information on the instant shaft position, relative to an initial position.
A pair of optical beams are used to sense the direction of rotation, wherein the path of the second beam is positioned in such a way as to produce a second electrical signal similar to the first one, but shifted in angular position by a quarter of the period of the first signal. Such two signals will be further said to be in quadrature.
An improvement of the above method consists of using a light beam that covers several of the sections with alternating optical properties, while placing a fixed mask between the rotating optical disc and the light sensor. The mask has a pattern identical to the disc pattern, and covers at least the surface of the light sensor. Whenever mask and disc patterns coincide, a maximum of light is transmitted whereas when the mask and disc and mask pattern are in opposite phase, light transmission is minimum. Thus the amount of light transmitted to the sensor and the amplitude of the electronic signal created by the sensor become a periodic function of the angular position.
There is a continuing trend of improving the precision and resolution of position feedback devices.
A first factor limiting precision is a small eccentric movement of the rotating optical disc, which is due to some mechanical tolerance. Since the two beams intersect the optical disc at defined positions on the disc, on one side of the shaft, a lateral movement of the shaft influences the amount of light that reaches the light detector, thus creating error in the position information. Such a lateral movement can be caused by tolerance in the roll bearing holding the shaft, or in the optical disc assembly.
A second factor is the precision of the pattern on the rotating disc. Irregularities in that pattern will generate unequal periods such that the optical disk will fail to indicate the angular position of the shaft. Since the two beams intersect the optical disc at a defined position on the disc, on one side of the shaft, irregularities of the disc pattern will influence the amount of light that reaches the light detector, introducing errors in the position information.
A third factor is the insufficient precision of the phase difference between the two signals in quadrature, this phase difference being obtained by a predetermined relative positioning of the two receptors, whose precision is limited.
In another aspect, when the resolution of an optical encoder is increased, the frequency of the electric signal generated is also increased. Such high frequencies become more difficult to transmit through wires and a problem of noise immunity arises. The wires must then be shielded, resulting in higher cost and reduced reliability of the system.
In some encoders, this difficulty is overcome by using a serial communication line between the encoder and the controller. However in this case an electronic circuit must be added to convert the electric signals into a serial code. Also a delay is introduced between the encoder position detection and the actual, position information read by the controller.
A further drawback of known Optical Encoders is the need for an electronic circuit inside the encoder itself. This electric circuit is most commonly used to shape the two electrical signals in square pulses. Such an electrical circuit requires power supply and thus additional wires to the encoder. Again the cable size and number of conductors is increased, resulting in a reduced reliability for example when used in an industrial environment. In addition the cost of the electronic interface and of assembling the interface inside the encoder is relatively high.
It is therefore desirable to design an Optical Encoder that overcomes the drawbacks of the commonly used encoders. It is desirable to have an Optical Encoder that requires no power supply and in which no delay is introduced in the position information. It is further desirable to have an optical encoder in which the position signal is a) immune to electrical noise, b) not sensitive to the mechanical eccentric movements of the disc during rotation, c) not sensitive to encoder disc mounting imprecision or to the irregularities of the encoder disc pattern and d) not sensitive to the mechanical vibrations of the disc.